In Single Photon Emission Computed Tomography (SPECT), a nuclear imaging technique, molecules labeled with a gamma-emitting radioisotope, commonly referred to as a tracer, are injected into a biological entity under study, e.g. a patient. By measuring the gamma rays originating from the decaying isotope using a radiation detector, the distribution of the labeled molecule can be determined. The tracer spatial distribution can be reconstructed by determining the direction of incidence of the photons on the detector. In order to determine this direction of incidence, a collimator may be positioned in front of the radiation detector to select a spatial angle of incidence. Such collimator may consist of a block of material having a high attenuation coefficient with respect to the emitted gamma rays. In this material, small holes are provided to allow incidence of photons from a limited acceptance angle. In order to reconstruct a 3D image of the radioisotope distribution, each point in the distribution is observed from a sufficient number of angles, in order to achieve sampling completeness. In methods known in the art, this is usually achieved by rotating the collimator and detector around the subject. This rotation may for example be halted at certain angular positions to acquire data at these predetermined angles. However, the rotation of the system is never perfect due to mechanical tolerances, which leads to errors in the reconstruction due to the incorrect calibration of the system after rotation. The mechanism responsible for the rotation can also be quite large and expensive, e.g. due to the required high precision positioning of the gamma-cameras. Furthermore, while the system is rotating, it may not be available to acquire useful data for the reconstruction. Since the system typically has to be rotated such as to sample a large number of angles to get sufficient sampling, this implies an extended scanning period.
SPECT systems are known in the art which avoid rotation of at least the gamma camera detector or the collimator.
For example, the use of a rotating annular or cylindrical collimator in combination with a stationary detector system is known in the art. A similar approach is known in the art that uses slant-hole collimators, which are rotated around the scanner's longitudinal axis as well as around a perpendicular axis, while the detectors remain stationary. Such rotation of the collimators around an axis perpendicular to the longitudinal axis can be particularly challenging technically. Furthermore, even though the detector remains stationary in these systems known in the art, the collimator comprises a high attenuation material, which implies a high mass density, and therefore such system still requires a large and expensive actuation system to provide the precisely controlled rotation of the collimator.
The use of multi-head synthetic collimation is also known in the art. Such systems may use a radial motion of the detectors or multiple detectors arranged behind each other so as to provide a stacked detector acquisition. However, such systems have the disadvantage of requiring additional detectors and/or additional means for actuation of the detectors, thus increasing the size and cost of the system.
The use of multi-pinhole collimators to perform stationary cardiac SPECT is also known. In such systems, the collimator and the detector can be stationary. However, the imaging volume does not provide full 360 degree coverage in such systems, such that the useable field of view for accurate reconstruction is limited. Particularly, the useable field of view for accurate reconstruction, e.g. which satisfies sampling completeness, is relatively small compared to the volume delimited by the collimators. Therefore, a translation along three orthogonal axes of the bed on which the subject is positioned may be required.
For example, cylindrical multi-pinhole collimators are known in the art having one row of pinholes surrounding the subject. Such systems may be used in, for example, brain imaging and small-animal SPECT. However, the number of sampled angles obtained by such systems, and thus the different views of the object available for image reconstruction, is limited.
The use of another type of cylindrical multi-pinhole collimators surrounding the object is also known, in which the pinholes focus on a small field of view from sufficient viewing angles, e.g. for small-animal SPECT imaging. The subject may be translated on a bed along three orthogonal axes to obtain sufficient data from the region of interest. However, the application of such a system is limited by the use of pinholes, and full angular sampling of a large portion of the subject can only be achieved through bed translations along a non-linear path.
For example, EP 2414864 discloses an interwoven multi-aperture collimator, which comprises a body including a plurality of apertures in a two-dimensional grid. The two-dimensional grid is selectively divided into at least a first and a second group of apertures, defining corresponding views of an object to be imaged. The first group is formed by interleaving or alternating rows of the grid, and the second group is formed by the rows adjacent to the rows of the first group. Each aperture in the first group is arranged in a first orientation angle with respect to the surface plane of the collimator body, and each aperture in the second group is arranged in a second orientation angle with respect to the surface plane of the collimator body such that the apertures of the first group are interwoven with the apertures of the second group. This collimator was designed, however, to be attached to a single radiation detection module and to image the object from a single angular position, and so even if there are multiple aperture groups defining different views of the object the angular sampling of any particular part of the object is limited.
US2009/001273 discloses a collimator for a SPECT system. The collimator comprises a plurality of individual collimating segments. Each of the collimating segments are identical and comprise a plurality of apertures whereby each of the apertures have the same orientation angle with respect to the surface plane of the collimating segment. In US2009/001273 the individual collimating segments are angularly displaced from one another about a common central axis. As a result of this rotation each collimating segment defines a different projection view along a different projection direction. Such a collimator has the disadvantage that the different collimating segments need to be attached, leading to a loss of valuable detector space and a decreased image quality. Furthermore, in order to obtain sufficient angular sampling, parts of such a collimator are quite far from the detector and the object to be imaged, which leads not only to an inefficient use of space but to great losses in image quality.